Considerable development has taken place in recent decades with regard to knee joint replacement. However, continuous review of clinical experience with unicompartmental meniscal knee implants has shown that problems can occur, particularly in the case of replacement of the lateral compartment. Whilst great success has been achieved with medial compartmental replacement, dislocation of the meniscal bearing in the lateral compartment remains a problem. The success rate of such replacements has been limited, due to the fact that the lateral soft tissues (principally the lateral collateral ligament and the ilio-tibial band) offer less certain resistance to distraction of the joint. There is also evidence to suggest that the lateral compartment may lift off under certain circumstances, such as during single leg stance, resulting in unequal loading of the joint. The lateral collateral ligament is a much slimmer structure than the medial collateral ligament and there is evidence to suggest that it is slack in the unloaded joint, except at extension. It therefore offers much less resistance to bearing dislocation or lift-off than do the more inextensible medial structures. On the lateral side, the tendon of the popliteus muscle passes across the postero-lateral corner of the joint. When the joint is replaced, the tendon can act to propel the meniscal bearing towards the intercondylar region where dislocation can occur.
The early total condylar prostheses designed in the 1970's sacrificed both cruciate ligaments as does their successor, the Insall-Burstein posterior stabilised prosthesis. Many of the prostheses designed in the 1980's sacrificed the anterior cruciate ligament (ACL) but allowed retention of the posterior cruciate ligament. Attempts to develop total joint prostheses where the ACL is also retained have generally been unsuccessful. The prosthetic components were either designed to constrain antero-posterior translational movements and inadequate steps were taken to prevent loosening, or the components were unconstrained relative to anterior/posterior translation and many wore out.
It has been observed that during flexion and extension of the knee joint, the contact points between the femoral and tibial condyles move in an antero-posterior direction. The femoral component moves on the tibial plateau in a posterior direction during flexion and in an anterior direction during extension. One of the problems caused by absence of the ACL is increased antero-posterior movement of the femoral condyle relative to the tibial condyle, which is responsible for further loosening of the tibial component and often leads to dislocation. A conventional tibial component will generally have a large central pin for location in the prepared end of the tibia. The ACL may be present in a diseased knee but is usually removed to improve access to the joint area during surgery, which is necessary to provide sufficient clearance to insert the tibial component and provide a site for the central pin.
Examination of records of the state of the ligaments at the time of knee-replacement surgery shows that in more than 50% of cases, both in osteoarthritis or in rheumatoid arthritis, all the ligaments, including the ACL, were found to be intact. Where present, these ligaments are generally sacrificed in the above surgical techniques. Although tibial components having a central cut-out slot of the type illustrated in FIG. 1, which can be inserted with the ACL in place, have been employed, there will still be a tendency for dislocation or lift-off in the lateral compartment.
In those cases where the ligaments are already absent, they may be reconstructed. Since the attempts of the 1960's, the practice of ACL reconstruction in young athletes, using muscle tendons as grafts, has become widespread and there is a large body of surgeons for whom this operation forms a substantial part of their practice. Although such routine reconstructions have usually only been performed in young persons, following injury involving ligament damage, there is considerable scope for reconstruction in the elderly patients who are the usual candidates for knee replacement, in cases where the ACL is absent.
From the foregoing it is apparent that a need exists for knee replacement prostheses which provide sufficient stability in the lateral compartment and which are suitable for joints with intact cruciate ligaments.